Although catalysts for the conversion of hydrocarbons have a tendency to deactivate, usually a catalyst's activity may be restored by one of a number of processes that are known generally as regeneration processes. Regeneration processes are extensively used. What specific steps comprise a regeneration process depends in part on the reason for the deactivation. For example, if the catalyst deactivated because coke deposits accumulated on the catalyst, regeneration usually includes removing the coke by burning. If the catalyst deactivated because a catalytic metal such as platinum becomes agglomerated, regeneration usually includes redispersing the metal by contacting the catalyst with oxygen and chlorine. If the catalyst deactivated because a catalytic promoter such as chloride becomes depleted, regeneration usually includes replenishing the promoter by contacting the catalyst with a chlorine-containing species, which are referred to herein as chloro-species. Operating conditions and methods for these regeneration processes are well known. Regeneration processes can be carried out in situ, or the catalyst may be withdrawn from the vessel in which the hydrocarbon conversion takes place and transported to a separate regeneration zone for reactivation. Arrangements for continuously or semicontinuously withdrawing catalyst particles from a reaction zone and for reactivation in a regeneration zone are well known.
Many of these regeneration processes share the common feature of contacting the catalyst in the presence of one or more chloro-species that restore the activity of the catalyst for use in the reaction zone. These chloro-species may be chemically or physically sorbed on the catalyst as chloride or may remain dispersed in a stream that contacts the catalyst. In many regeneration processes, however, a flue gas stream containing the chloro-species is vented from the regeneration process. Several methods have been used for preventing contamination of the flue gas stream with the chloro-species and minimizing the release of the chloro-species in the flue gas stream from the regeneration process. Emissions of chloro-species, apart from the effect of the loss of chloride on the catalyst, pose environmental concern. The loss of chloride usually causes temporary deactivation that can be reversed by adding make-up chloride to the catalyst. The environmental concerns can be abated either by scrubbing the flue gas stream with an aqueous, basic solution that neutralizes the chloro-species or by adsorbing the chloro-species on an adsorbent. Scrubbing and adsorption are the two methods that are typically used when chloro-species are vented during regeneration of reforming catalysts and of catalysts for other hydrocarbon conversion processes, such as dehydrogenation, isomerization, alkylation, and transalkylation.
Although these two methods--scrubbing and adsorption--for decreasing the venting of chloro-species during catalyst regeneration are useful, they are also expensive to build and troublesome to operate. On the one hand, by introducing an aqueous solution into the process, scrubbing can actually increase the risk of downstream corrosion unless the alkalinity of the aqueous solution is carefully controlled. Moreover, because the aqueous solution must be replaced periodically, scrubbing gives rise to significant costs for supplying fresh solution and for disposing of the spent solution. On the other hand, although adsorption does not involve the introduction of an aqueous stream, the adsorbent also must be replaced periodically, and the cost of replacement of the adsorbent, including the cost of disposing of spent adsorbent, can far exceed the cost of replacement of the aqueous solution in scrubbing.
The problem of adsorbent replacement is compounded by water in the flue gas stream, and as a result traditional adsorbents are not economically viable for adsorbing chloro-species from flue gas streams. In order to be economically viable, an adsorbent, while removing a high proportion of the chloro-species from the flue gas stream, must adsorb typically from 7 to 8 percent of its weight in chloride. In order to adsorb that much chloride, the flue gas must have a low water content, typically less than 0.01 mol-% water. Water competes with chloro-species for adsorption sites on the adsorbent, and by occupying sites that would otherwise be occupied by chloro-species, water hinders the adsorption of chloride and hastens replacement of the adsorbent. Thus, if the flue gas has a high water content, the adsorbent adsorbs too much water and is incapable of adsorbing a viable amount of chloride. Because water is a common by-product of coke combustion as a result of the hydrogen-containing compounds typically found in coke, flue gas streams often do have a high water content, typically from 1 to 10 mol-%. As a consequence, unless the flue gas is dried an adsorbent will adsorb only one-third to one-half of the weight of chloride required for economic viability. This, in turn, doubles or triples the frequency of adsorbent replacement, thereby making traditional adsorbents uneconomical. Although in theory the adsorption of water can be mitigated by drying the flue gas stream prior to adsorbing the chloro-species, in fact a drier is costly as well as impractical because chloro-species such as hydrogen chloride tend to degrade most desiccants.
Thus, a process is sought for removing hydrogen chloride and other chloro-species from the flue gas streams of catalyst regeneration processes without the need for aqueous solutions, adsorbents, and desiccants.